Radio receivers are used to receive an incoming radio frequency (RF) signal, process, and downconvert the signal to a lower frequency, e.g., an intermediate frequency (IF) or a baseband frequency, where the signal can be further processed to thus output an audio signal. In typical receivers, a signal path includes multiple components, both in an RF portion of the signal path as well as lower frequency portions of the signal path such as an IF and/or baseband portions. Because an incoming signal may be of relatively low power, some of these components may provide gain to the signal. Many receivers include gain elements both at RF as well as at lower frequencies.
To control these gain elements, many receivers include some type of automatic gain control (AGC) circuitry. AGC circuitry is often used in receivers for maximizing the usable range of input signal levels to a receiver signal path. Such gain control circuitry typically analyzes the level of an incoming signal such as via a peak detection and acts to automatically adjust the gain based on the detection. Other systems use analog circuitry that estimates the root mean square (RMS, also referred to as average) value of the incoming signal. However, both peak detectors and RMS detectors are relatively complicated devices and have high levels of tolerance requirements.
Typically the gain control in such AGC circuits occurs on a schedule determined by analog averaging of the detector signals, or on a relatively simple set of fixed rules applied to the detector signals. While such AGC schemes can work well in many situations, they suffer from drawbacks, including excessive power consumption, possible signal degradation due to unnecessary updates to the gain control elements, poor use of the dynamic range of the signal path circuits, and so forth. An additional complication is that different circuit elements respond differently to overload conditions, with open-loop circuits such as low noise amplifiers (LNAs) and mixers typically showing more gradual degradation as overload level increases, while closed-loop circuits such as programmable gain amplifiers (PGAs) typically show very abrupt and rapid degradation as overload level increases. Getting the best performance from a signal chain therefore requires an AGC scheme that is aware of these characteristics and can adjust gains appropriately. Furthermore, for multi-band receivers, because of the different types of encodings and modulation schemes for different bands, a single type of detector may not be applicable to multiple bands.
For example, for an FM band a constant envelope signal is received and thus has a relatively low (or zero) peak-to-average ratio (PAR). Therefore for receiving an FM signal an AGC scheme based on peak level will perform relatively similarly to an AGC scheme based on RMS level. However, certain digital bands such as digital audio broadcast (DAB) radio have an almost Gaussian distribution with a high PAR value. In this case an AGC scheme based on peak level will perform very differently from an AGC scheme based on RMS level. Accordingly, a single gain control hardware and algorithm for multiple bands can leave headroom or suffer overloading of a receiver.
For example, if a gain control system designed for an FM signal based on peak level is used for DAB reception, a very low range RMS signal will occur, losing dynamic range of the signal chain open-loop circuits. If the FM gain control is based on RMS signal level, then in DAB reception, very high range peak signals will occur, causing severe overload of the signal chain closed-loop circuits.
In contrast, if a gain control system designed for DAB signals based on peak level is used for FM reception, a very high range RMS signal will occur, causing severe overload of the signal chain open-loop circuits. If the DAB gain control is based on RMS signal level, then in FM reception very low range peak signals will occur, losing dynamic range of the signal chain closed-loop circuits.